The proposed studies extend earlier research on the neuroactive amino acid glycine, the chemical messenger used by a group of 12 identified giant neurons (R3-R14) in the invertebrate Aplysia. These cells innervate vascular smooth muscle and glycine apparently acts as a neuromodulator at those neuromuscular junctions, enhancing the muscle contractions. The advantages of large identified cell bodies located centrally, axons that project into the periphery, isolable terminals, and viability of these tissues in vitro make the R3-R14 system a unique prototype for basic cellular studies on glycine as a neural messenger. Major research objectives include study of the kinetics of glycine movement (by axonal transport) to and from nerve terminals, the biological significance of the retrograde axonal transport, the subcellular localization of transported glycine, and the release of glycine from the R3-R14 terminals. Experiments addressing these objectives are done in a chamber in which the cell bodies (in the ganglion), axons (in the branchial nerve), and terminals (near the heart and on major arteries) are isolated from each other for separate treatment with chemically modified media. 3H-glycine is used to incubate portions of the preparation; its subsequent movement and release is quantified by scintillation spectrometry and autoradiography. Differential centrifugation and high resolution autoradiography is used to determine the subcellular localization of glycine. The significance of the proposed work lies first in its general applicability to the intracellular supply, movement, and release of neural messengers. Second, our recent description of a novel use for glycine - as a modulator of smooth muscle contraction - suggests that this amino acid has diverse messenger functions. The proposed work may thus contribute to the development of a new model system for studying nerve-muscle relationships. Third, studies proposed on the release of glycine would further support the role of glycine as the chemical messenger in cells R3-R14.